The human vertebral column is a vital part of the human physiology that houses and protects the spinal cord, and provides structural support for the body. In a typical human, the vertebral column is made up of twenty-four articulating vertebrae and nine fused vertebrae, and is generally divided into several regions, including the cervical, thoracic, sacral, and coccygeal regions.
While variations exist between each vertebra depending on its location and region, vertebrae generally consist of a body, pedicles, a lamina, a spinous process, transverse processes, facet joints, and a spinal canal, each of which play a pivotal role in providing the overall supportive and protective functionality of the vertebral column. Of these features, the vertebral body is of particular importance in providing support. The vertebral body is the largest portion of the vertebra, provides an attachment point of intervertebral discs, protects the spinal cord, and bears the majority of the load of the vertebra.
Each vertebra is separated from an adjacent vertebra by an intervertebral disc, a cartilaginous joint that acts as a ligament to hold the vertebrae together. A disc consists of an outer annulus fibrosus which surrounds the inner nucleus pulposus. The annulus fibrosus consists of several layers of fibrocartilage which contain the nucleus pulposus and distribute pressure evenly across the disc. The nucleus pulposus contains loose fibers suspended in a mucoprotein gel. The nucleus of the disc acts as a shock absorber, absorbing the impact of the body's daily activities and keeping the two vertebrae separated.
While the interverbral disc protects adjacent vertebral bodies from impact or contact, various disorders may comprise the structure of the discs and negatively impact their functionality. For example, due to age, the nucleus pulposus may dehydrate and deform, or the annulus fibrosus may weaken and become more prone to tearing. Discs may also be damaged through trauma, resulting in undesirable bulging or loss of nucleus pulposus through a fissure. These disc disorders may diminish a disc's ability to absorb shock and transfer loads, or may cause adjacent vertebrae to contact, possibly resulting in acute or chronic pain for those suffering from these disorders.
To restore the functionality of a damaged or degenerated intervertebral disc, a common approach includes performing a discectomy to remove compromised material from within the intervertebral disc, and subsequently implanting a prosthesis in the void space created during the discectomy. The primary intention of these procedures is to ameliorate back pain by interrupting the vicious cycle that arises from abnormal spinal biomechanics and spinal instability, and by disrupting the cascade of reactive and degenerative processes of the bony and soft tissue. A secondary benefit is to limit the collateral damage to the spinal soft tissue envelope that is typical of traditional open spinal surgery and minimally invasive spinal surgery, thus diminishing postoperative pain and allowing earlier recovery.
So far, the methods and instrumentation required to achieve these goals have not been adequately developed or commercially available due to several deficiencies. First, existing methods and instrumentation have largely focused on total disc replacement, where the entirety of an intervertebral disc is removed and is replaced with a hinge-based prosthesis or a single-chambered disc-shaped inflatable structure. In these implementations, no attempt is made to preserve the annular fibrosis, which may be healthy despite degradation to the nucleus pulposus. Second, there is no existing method for the removal and replacement of intervertebral material, and the preservation of the annular fibrosis, that is performed entirely percutaneously.
Ideally, the treatment of intervertebral discs would involve a minimally invasive procedure, such that the discectomy and implantation process minimizes the disturbance of healthy surrounding tissue. Likewise, the tools and implants used during this process should be capable of minimally invasive deployment and operation. The implant should provide sufficient structural support to restore the functionality of an intervertebral disc, and should ideally preserve a significant degree of articulation freedom between vertebrae. The implant should also be resilient against sudden physical shocks and others external forces, such that it can withstand stresses seen during normal patient movement.